This invention relates to thyristor devices and in particular, a thyristor device which does not require a high reverse breakdown voltage.
As disclosed in U.S. Pat. No. 3,538,401 to Chang K. Chu issued on Nov. 3, 1970, a type of thyristor is well known wherein an N-type emitter layer, a P-type base layer, an N-type low impurity concentration layer, an N-type base layer of an impurity concentration higher than that of the N-type low impurity concentration layer, and a P-type emitter layer are formed in this order. When a voltage is applied across the P-type emitter layer and the N-type emitter layer of this type of thyristor with the positive polarity for the P-type emitter layer and the negative polarity for the N-type emitter layer, the applied voltage is mostly concentrated across a PN junction at the interface between the P-type base layer and the N-type low impurity concentration layer. This voltage is reverse to the PN junction and depletion layers are then created on opposite sides of the PN junction.
Since these depletion layers on the opposite sides tend to so spread as to equalize electric charges in the two depletion layers, the formation of the depletion layer is more extensive in the N-type low impurity concentration layer of a lower impurity concentration than in the P-type base layer.
Spreading of the depletion layer is, however, limited by the N-type base layer which is higher in impurity concentration than the N-type low impurity concentration layer. As a result, this depletion layer reaches a PN junction formed between the N-type base layer and the P-type emitter layer, thereby increasing the break-over voltage, i.e., the so-called punch-through voltage and hence the forward blocking voltage.
Although a high impurity concentration in the N-type base layer is followed by the increased punch-through voltage and the correspondingly increased forward blocking voltage of the thyristor, the forward on-state voltage when the thyristor is in conduction is also increased to thereby increase power loss during the conduction state. This is because the injection efficiency of minority carriers from the P-type emitter layer to the N-type base layer is lowered. Therefore, the impurity concentration in the N-type base layer is limited to 5.times.10.sup.16 atoms/cm.sup.3 at the PN junction portion between the P-type emitter and N-type base layers, preventing a sufficiently high forward blocking voltage of the thyristor from being obtained.